Motor-driven or motor-assisted bicycles are known wherein batteries are carried and an electric motor is mounted to the bicycle so as to provide a motive force to the bicycle. Normally, the principal motive force for the bicycle is still human pedalling power, and the motor assists this human pedalling power. Various driving devices have been proposed for motor-driven bicycles. Such devices include driving devices in which a DC electric motor is built into one of the hubs of the bicycle as disclosed in U.S. Pat. No. 5,560,442. Such devices are advantageous in that a compact driving mechanism can be constructed.
It is generally desirable to provide components of a hub electric motor unit that can be manufactured at low manufacturing costs and that can efficiently be assembled. It is further generally desirable to provide components of a modulated pole machine that fulfill one or more of the following criteria: high stability, high durability, high resistance against stress, low weight, high performance numbers or values of the hub electric motor unit, such as torque pr. volume and/or torque pr. current.